Field of the Invention
The present invention relates to a planar optical waveguide device, a polarization multiplexing 4-value phase modulator, a coherent receiver, and a polarization diversity used in optical fiber communication.
Description of the Related Art
In recent years, the amount of information transmitted through optical communications has been steadily increasing. To cope with the increase in the amount of information, measures such as increasing a signal speed or increasing the number of channels based on wavelength multiplexing communication have been developed. Particularly, in the next generation 100 Gbps digital coherent transmission technology for high-speed information communication, in order to double the amount of information per unit time, a polarization multiplexing method for carrying information in two polarizations where electric fields are orthogonal to each other is used.
However, in a modulation method for high-speed communication including polarization multiplexing, an optical modulator having a complicated structure is necessary, which can cause a problem in that the size of an apparatus becomes large and the manufacturing cost increases. In order to solve these problems, research regarding an optical modulator based on a planar optical waveguide using silicon having merits such as easy processing, size reduction due to integration, cost reduction due to mass production, or the like has been performed.
However, polarization multiplexing in such a planar optical waveguide can have the following problems. In general, a planar optical waveguide has an asymmetric shape in a width direction parallel to a substrate and in a height direction perpendicular to the substrate. Thus, a characteristic such as an effective refractive index varies between two types of polarization modes of a mode (hereinafter, referred to as a TE mode) in which a width-directional electric field component is a main component and a mode (hereinafter, referred to as a TM mode) in which a height-directional electric field component is a main component. In many cases, TE0 and TM0 among the two polarization modes are frequently used. Here, TE0 is a mode in which an effective refractive index is largest in the TE mode, and TM0 is a mode in which an effective refractive index is largest in the TM mode.
In a case where an optical modulation operation is performed with respect to these polarization modes in which characteristics are different from each other, it is difficult to perform the optical modulation operation by only using a single planar optical waveguide device. Thus, it is necessary to provide a planar optical waveguide device optimally designed for each polarization mode, which causes a problem in that a large amount of effort is necessary for development of a planar optical waveguide device.
In order to solve the problems, a method for using TE0 as input light to a planar optical waveguide device optimally designed for TE0 and polarization-converting output light thereof into TM0 may be used. Here, the “polarization conversion” refers to conversion from TE0 to TM0 or from TM0 to TE0. In order to perform the optical modulation operation, it is necessary to provide a planar optical waveguide device that performs polarization conversion on a substrate.
In order to perform polarization conversion on a substrate, a technique that combines a conversion between TE0 and TE1 and a conversion between TE1 and TM0 may be used. The invention pays attention to the conversion between TE0 and TE1 among these conversions. Here, TE1 represents a mode having a second largest effective refractive index in the TE mode.
As a related art relating to an optical waveguide device having a function for the conversion between TE0 and TE1, there is an optical waveguide device disclosed in Daoxin Dai and John E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires.” Optics Express, Vol. 19, Issue. 11, pp. 10940-10949 (2011) (hereinafter, referred to as Non-Patent Document 1).
FIGS. 69A and 69B are diagrams illustrating an optical waveguide device which is a model of a structure disclosed in Non-Patent Document 1. FIG. 69A is a plan view thereof, and FIG. 69B is a sectional view thereof.
The optical waveguide device includes core portions 81 and 82, and a cladding 15. The cladding 15 includes a lower cladding 7 and an upper cladding 6.
The core portions 81 and 82 are linear waveguides, and are disposed in parallel to form a directional coupler. In the directional coupler, TE0 of the core portion 81 and TE1 of the core portion 82 are coupled to perform mode conversion.
In order to efficiently perform mode conversion in the directional coupler, it is necessary to maintain effective refractive indexes in TE0 and TE1 at the same level. Thus, a waveguide structure is adjusted according to each mode.
In this optical waveguide device, in order to maintain the effective refractive indexes in TE0 and TE1 at the same level, the widths of the core portions 81 and 82 are adjusted. Since the widths of the core portions 81 and 82 are different from each other, such a directional coupler is referred to as an “asymmetric directional coupler”.
However, the above-described optical waveguide device combines different modes. Thus, even if the condition for “maintaining effective refractive indexes in TE0 and TE1 at the same level” with respect to a specific wavelength is satisfied by adjustment of a waveguide structure (adjustment of the width of a core portion, or the like), a wavelength may deviate from the specific wavelength. Further, in a case where a waveguide structure is changed due to a manufacturing error, deviation occurs between effective refractive indexes of the two modes. Accordingly, conversion efficiency may be lowered.
Accordingly, in the related art, there are problems in that a wavelength band for allowing highly efficient conversion is narrow and stability against a manufacturing error is weak.
Hereinafter, the problems will be described using an asymmetric directional coupler in the related art shown in FIGS. 69A and 69B as an example.
In this example, core portions 81 and 82 are formed of Si (having a refractive index of 3.48), and an upper cladding 6 and a lower cladding 7 are formed of SiO2 (having a refractive index of 1.44). The heights of the core portions 81 and 82 are 220 nm. A gap between the core portions 81 and 82 is 200 nm.
A waveguide which guides light in TE0 which is a mode conversion target and has the core portion 81 having a smaller width is referred to as “waveguide 1”, and a waveguide which guides light in TE1 and has the core portion 82 having a larger width is referred to as “waveguide 2”.
The width of the core portion 81 is 400 nm. Here, at a wavelength of 1580 nm, the width of the core portion 82 is set to 838 nm so that effective refractive indexes in TE0 of the core portion 81 and TE1 of the core portion 82 are at the same level. Calculation results of the effective refractive indexes are shown in Table 1. A finite element method (FEM) is used for the calculation.
TABLE 1TE0 of waveguide 1TE1 of waveguide 2Effective refractive index2.1788182.178940
A conversion efficiency of the asymmetric directional coupler is as follows. Here, a conversion efficiency T is a ratio of power of output TE1 to power of input TE0.[Expression 1]T=F sin2(qL)  (1)
Here, F and q are expressed as the following expressions, respectively.
                    [                  Expression          ⁢                                          ⁢          2                ]                                                            F        =                  1                      1            +                                          (                                  δ                  χ                                )                            2                                                          (        2        )                                [                  Expression          ⁢                                          ⁢          3                ]                                                            q        =                                            χ              2                        +                          δ              2                                                          (        3        )            
Here, δ is expressed as the following expression.
                    [                  Expression          ⁢                                          ⁢          4                ]                                                            δ        =                              π            λ                    ⁢          Δ          ⁢                                          ⁢          N                                    (        4        )            
Here, L represents the length of an asymmetric directional coupler in a light propagation direction, ΔN represents a difference between effective refractive indexes (difference between effective refractive indexes in Table 1) in TE0 of the waveguide 1 and TE1 of the waveguide 2 in a case where two waveguides are independently present, and λ represents a wavelength. Further, χ represents the strength of coupling of two waveguides, and is referred to as a coupling coefficient.
In the asymmetric directional coupler, even if effective refractive indexes of two modes which are coupling targets match each other by adjusting a waveguide structure such as the width of a core portion or the like at a certain wavelength (1580 nm in this example), if the wavelength is changed, deviation occurs in the effective refractive indexes.
This problem does not occur in a symmetric directional coupler that has the same heights and widths in two cores and handles coupling of the same modes but occurs in an asymmetric directional coupler that handles coupling of different modes.
FIG. 70 is a diagram illustrating a relationship between a wavelength and an absolute value of ΔN in an optical waveguide device in this example. It can be understood from FIG. 70 that the absolute value of ΔN becomes larger as the wavelength deviates farther from 1580 nm.
Since the conversion efficiency T is lowered according to the deviation of the wavelength, from Expression (1), (2), and (4), highly efficient conversion is not preferable in a wide wavelength band.
Then, the conversion efficiency with respect to the wavelength (1520 nm to 1640 nm) is calculated based on Expression (1) to Expression (4). The result is shown in FIG. 71. Here, L in Expression (1) is a value in which a minimum value of the conversion efficiency in the wavelength band of 1520 nm to 1640 nm becomes a maximum, and in this case, L is 16.1 μm.
Referring to FIG. 71, the conversion efficiency becomes lower as the wavelength becomes more distant from the vicinity of 1580 nm, and becomes equal to or greater than approximately −0.94 dB in the wavelength band of 1520 nm to 1640 nm. This is because the absolute value of ΔN increases with respect to the above-described wavelength.
Subsequently, a relationship between a manufacturing error and conversion efficiency will be described. If a waveguide structure is changed, the level of light confinement is changed, and an effective refractive index associated therewith is changed. Thus, even if a waveguide structure is designed so that effective refractive indexes of two modes which are coupling targets are at the same level at a certain wavelength, the waveguide structure is changed due to a manufacturing error, and the effective refractive indexes of two modes deviate from each other.
Thus, the conversion efficiency is lowered as in the above description regarding the wavelength dependency.
In order to confirm this problem, a manufacturing error of the width of a core portion generated due to lithography or etching will be described as an example.
Normally, a manufacturing error locally occurs in two core portions 81 and 82 by the same amount (δ), as shown in FIG. 72, with respect to design values of the widths of the core portions (the widths of core portions regulated by a mask, for example, W81 and W82 in FIG. 72). In this example, it is assumed that that positions on both side edges of respective cores are changed inward or outward by δ/2, respectively.
Hereinafter, a case where a manufacturing error δ (=−30 nm) occurs with respect to the core portion 81 (design value: width of 400 nm) and the core portion 82 (design value: width of 838 nm) of the optical waveguide device shown in FIGS. 69A and 69B is considered. FIG. 73 is a diagram illustrating a relationship between a wavelength and an absolute value of ΔN.
It can be understood from FIG. 73 that effective refractive indexes in TE0 of the core portion 81 and TE1 of the core portion 82 significantly deviate from each other, and thus, the absolute value of ΔN becomes large. The conversion efficiency is calculated based on this result. L employs the above-described value (L=16.1 μm). A result thereof is shown in FIG. 74.
It can be understood from FIG. 74 that since the absolute value of ΔN becomes large due to the manufacturing error, the conversion efficiency is significantly lowered. Specifically, the conversion efficiency becomes equal to or greater than about −5.16 dB at 1580 nm, and becomes equal to or greater than −7.32 dB in a range of 1520 nm to 1640 nm. In this view, it can be said that an asymmetric directional coupler is weak against a manufacturing error.
In this way, in an optical waveguide device including an asymmetric directional coupler in the related art, there are problems in that a wavelength band in mode conversion is narrow and stability against a manufacturing error is weak.
In consideration of the above-described problems, an object of the invention is to provide a planar optical waveguide device capable of securing high conversion efficiency in a wide wavelength band, and securing efficient mode conversion even in a case where a waveguide structure is changed due to a manufacturing error.